Evolving Viscous Anisotropy in the Upper Mantle and Its Geodynamic Implications

Abstract

Asthenospheric shear causes some minerals, particularly olivine, to develop anisotropic textures that can be detected seismically. In laboratory experiments, these textures are also associated with anisotropic viscous behavior, which should be important for geodynamic processes. To examine the role of anisotropic viscosity for asthenospheric deformation, we developed a numerical model of coupled anisotropic texture development and anisotropic viscosity, both calibrated with laboratory measurements of olivine aggregates. This model characterizes the time-dependent coupling between large-scale formation of lattice-preferred orientation (i.e., texture) and changes in asthenospheric viscosity for a series of simple deformation paths that represent upper mantle geodynamic processes. We find that texture development beneath a moving surface plate tends to align the a axes of olivine into the plate motion direction, which weakens the effective viscosity in this direction and increases plate velocity for a given driving force. Our models indicate that the effective viscosity increases for shear in the horizontal direction perpendicular to the a axes. This increase should slow plate motions and new texture development in this perpendicular direction and could impede changes to the plate motion direction for tens of millions of years. However, the same well-developed asthenospheric texture may foster subduction initiation perpendicular to the plate motion and deformations related to transform faults, as shearing on vertical planes seems to be favored across a sublithospheric olivine texture. These end-member cases examining shear deformation in the presence of a well-formed asthenospheric texture illustrate the importance of the mean olivine orientation, and its associated viscous anisotropy, for a variety of geodynamic processes. Plain Language Summary The uppermost layer of Earth's mantle, the asthenosphere, experiences large deformations due to a variety of tectonic processes. During deformation, grains of olivine, the main rock-forming mineral in the asthenosphere, rotate into a preferred direction parallel to the deformation, developing a texture that can affect the response of the asthenosphere to tectonic stresses. Laboratory measurements show that the deformation rate depends on the orientation of the shear stress relative to the olivine texture. We use numerical models to apply the findings of the laboratory measurements to geodynamic situations that are difficult to simulate in a laboratory. These models track the development of olivine texture and its directional response to shear stress, which are highly coupled. Our results suggest that anisotropic viscosity in the asthenosphere can significantly affect the motions of tectonic plates, as plate motion in a continuous direction should become faster, while abrupt changes in the direction of plate motion should meet high resistance in the underlying asthenosphere. We suggest that olivine textures in the asthenosphere play a critical role in upper mantle dynamics.

Extrinsic Anisotropy of Two‐Phase Newtonian Aggregates: Fabric Characterization and Parameterization

“…The weakening is thus about a half of the predicted by SPO in our models, while the directional variations linked to LPO claimed by Király et al. ( 2020 ) can be up to two larger than what observed in our two‐phase aggregates and DEM models (Figure 7 ). This implies that an aggregate with mechanically anisotropic crystals should be more susceptible to changes in flow directions than when only isotropic SPO‐related fabrics are present.…”

“…Recent numerical studies demonstrated that anisotropy related to lattice preferred orientation (LPO) in olivine crystals can yield a weakening of about ω = 30% in the shear direction, and up to one order of magnitude viscosity variations depending on the dominant slip system (Király et al, 2020). The weakening is thus about a half of the predicted by SPO in our models, while the directional variations linked to LPO claimed by Király et al (2020) can be up to two larger than what observed in our two-phase aggregates and DEM models (Figure 7). This implies that an aggregate with mechanically anisotropic crystals should be more susceptible to changes in flow directions than when only isotropic SPO-related fabrics are present.…”

On the Relation Between Basal Erosion of the Lithosphere and Surface Heat Flux for Continental Plume Tracks

“…Thus, only lateral heterogeneities in the lithospheric structure, which may include local weak zones or a pre‐existing variations in lithosphere thickness, are expected to have an impact on lithospheric thinning and associated surface heat flux. In addition, anisotropic lithosphere viscosity may affect the position and dynamics of dripping instabilities (Király et al., 2020; Lev & Hager, 2008).…”

On the relation between basal erosion of the lithosphere and surface heat flux for continental plume tracks

“…Thus, only lateral heterogeneities in the lithospheric structure, which may include local weak zones or a pre-existing variations in lithosphere thickness, are expected to have an impact on lithospheric thinning and associated surface heat flux. In addition, anisotropic lithosphere viscosity may affect the position and dynamics of dripping instabilities (Király et al, 2020;Lev & Hager, 2008). 2020)) for 150 km thick cratons, we would expect maximum relative heat flux anomalies of about 20% and 60%, respectively (Figure 4).…”